Integers

For general purpose, nginx code uses the following two integer types ngx_int_t and ngx_uint_t which are typedefs for intptr_t and uintptr_t.

Common return codes

Most functions in nginx return the following codes:

NGX_OK — operation succeeded

NGX_ERROR — operation failed

NGX_AGAIN — operation incomplete, function should be called again

NGX_DECLINED — operation rejected, for example, if disabled in configuration. This is never an error

NGX_BUSY — resource is not available

NGX_DONE — operation done or continued elsewhere. Also used as an alternative success code

NGX_ABORT — function was aborted. Also used as an alternative error code

Error handling

For getting the last system error code, the ngx_errno macro is available. It’s mapped to errno on POSIX platforms and to GetLastError() call in Windows. For getting the last socket error number, the ngx_socket_errno macro is available. It’s mapped to errno on POSIX systems as well, and to WSAGetLastError() call on Windows. For performance reasons the values of ngx_errno or ngx_socket_errno should not be accessed more than once in a row. The error value should be stored in a local variable of type ngx_err_t for using multiple times, if required. For setting errors, ngx_set_errno(errno) and ngx_set_socket_errno(errno) macros are available.

The values of ngx_errno or ngx_socket_errno can be passed to logging functions ngx_log_error() and ngx_log_debugX(), in which case system error text is added to the log message.

Strings

Overview

The len field holds the string length, data holds the string data. The string, held in ngx_str_t, may or may not be null-terminated after the len bytes. In most cases it’s not. However, in certain parts of code (for example, when parsing configuration), ngx_str_t objects are known to be null-terminated, and that knowledge is used to simplify string comparison and makes it easier to pass those strings to syscalls.

A number of string operations are provided in nginx. They are declared in src/core/ngx_string.h. Some of them are wrappers around standard C functions:

ngx_strcmp()

ngx_strncmp()

ngx_strstr()

ngx_strlen()

ngx_strchr()

ngx_memcmp()

ngx_memset()

ngx_memcpy()

ngx_memmove()

Some nginx-specific string functions:

ngx_memzero() fills memory with zeroes

ngx_cpymem() does the same as ngx_memcpy(), but returns the final destination address This one is handy for appending multiple strings in a row

ngx_movemem() does the same as ngx_memmove(), but returns the final destination address.

ngx_strlchr() searches for a character in a string, delimited by two pointers

Some case conversion and comparison functions:

ngx_tolower()

ngx_toupper()

ngx_strlow()

ngx_strcasecmp()

ngx_strncasecmp()

Formatting

A number of formatting functions are provided by nginx. These functions support nginx-specific types:

ngx_sprintf(buf, fmt, …)

ngx_snprintf(buf, max, fmt, …)

ngx_slpintf(buf, last, fmt, …)

ngx_vslprint(buf, last, fmt, args)

ngx_vsnprint(buf, max, fmt, args)

The full list of formatting options, supported by these functions, can be found in src/core/ngx_string.c. Some of them are:

Numeric conversion

ngx_atoi(line, n) — converts a string of given length to a positive integer of type ngx_int_t. Returns NGX_ERROR on error

ngx_atosz(line, n) — same for ssize_t type

ngx_atoof(line, n) — same for off_t type

ngx_atotm(line, n) — same for time_t type

ngx_atofp(line, n, point) — converts a fixed point floating number of given length to a positive integer of type ngx_int_t. The result is shifted left by points decimal positions. The string representation of the number is expected to have no more than points fractional digits. Returns NGX_ERROR on error. For example, ngx_atofp(“10.5”, 4, 2) returns 1050

Regular expressions

The regular expressions interface in nginx is a wrapper around the PCRE library. The corresponding header file is src/core/ngx_regex.h.

To use a regular expression for string matching, first, it needs to be compiled, this is usually done at configuration phase. Note that since PCRE support is optional, all code using the interface must be protected by the surrounding NGX_PCRE macro:

The arguments of ngx_regex_exec() are: the compiled regular expression re, string to match s, optional array of integers to hold found captures and its size. The captures array size must be a multiple of three, per requirements of the PCRE API. In the example, its size is calculated from a total number of captures plus one for the matched string itself.

The ngx_regex_exec_array() function accepts the array of ngx_regex_elt_t elements (which are just compiled regular expressions with associated names), a string to match and a log. The function will apply expressions from the array to the string until the match is found or no more expressions are left. The return value is NGX_OK in case of match and NGX_DECLINED otherwise, or NGX_ERROR in case of error.

Time

The ngx_time_t structure represents time split into seconds and milliseconds with specification of GMT offset:

The ngx_tm_t is an alias for struct tm on UNIX platforms and SYSTEMTIME on Windows.

To obtain current time, usually it is enough to access one of available global variables, representing the cached time value in desired format. The ngx_current_msec variable holds milliseconds elapsed since Epoch and truncated to ngx_msec_t.

The ngx_time() and ngx_timeofday() macros returning current value of seconds are a preferred way to access cached time value.

To obtain the time explicitly, ngx_gettimeofday() may be used, which updates its argument (pointer to struct timeval). Time is always updated when nginx returns to event loop from system calls. To update the time immediately, call ngx_time_update(), or ngx_time_sigsafe_update() if you need it in the signal handler context.

The following functions convert time_t into broken-down time representation, either ngx_tm_t or struct tm for those with libc prefix:

ngx_gmtime(), ngx_libc_gmtime() — result time is UTC

ngx_localtime(), ngx_libc_localtime() — result time is relative to the timezone

ngx_array_push_n(a, n) adds n tail elements and returns pointer to the first one

If currently allocated memory is not enough for new elements, a new memory for elements is allocated and existing elements are copied to that memory. The new memory block is normally twice as large, as the existing one.

s = ngx_array_push(a);
ss = ngx_array_push_n(&b, 3);

List

List in nginx is a sequence of arrays, optimized for inserting a potentially large number of items. The list type is defined as follows:

Initially, a list must be initialized by calling ngx_list_init(list, pool, n, size) or created by calling ngx_list_create(pool, n, size). Both functions receive the size of a single item and a number of items per list part. The ngx_list_push(list) function is used to add an item to the list. Iterating over the items is done by direct accessing the list fields, as seen in the example:

The primary use for the list in nginx is HTTP input and output headers.

The list does not support item removal. However, when needed, items can internally be marked as missing without actual removing from the list. For example, HTTP output headers which are stored as ngx_table_elt_t objects, are marked as missing by setting the hash field of ngx_table_elt_t to zero. Such items are explicitly skipped, when iterating over the headers.

Queue

Queue in nginx is an intrusive doubly linked list, with each node defined as follows:

To deal with a tree as a whole, you need two nodes: root and sentinel. Typically, they are added to some custom structure, thus allowing to organize your data into a tree which leaves contain a link to or embed your data.

The insert_value_function is a function that is responsible for traversing the tree and inserting new values into correct place. For example, the ngx_str_rbtree_insert_value functions is designed to deal with ngx_str_t type.

Hash

Hash table functions are declared in src/core/ngx_hash.h. Exact and wildcard matching is supported. The latter requires extra setup and is described in a separate section below.

To initialize a hash, one needs to know the number of elements in advance, so that nginx can build the hash optimally. Two parameters that need to be configured are max_size and bucket_size. The details of setting up these are provided in a separate document. Usually, these two parameters are configurable by user. Hash initialization settings are stored as the ngx_hash_init_t type, and the hash itself is ngx_hash_t:

The key is a pointer to a function that creates hash integer key from a string. Two generic functions are provided: ngx_hash_key(data, len) and ngx_hash_key_lc(data, len). The latter converts a string to lowercase and thus requires the passed string to be writable. If this is not true, NGX_HASH_READONLY_KEY flag may be passed to the function, initializing array keys (see below).

The hash keys are stored in ngx_hash_keys_arrays_t and are initialized with ngx_hash_keys_array_init(arr, type):

The second parameter can be either NGX_HASH_SMALL or NGX_HASH_LARGE and controls the amount of preallocated resources for the hash. If you expect the hash to contain thousands elements, use NGX_HASH_LARGE.

The ngx_hash_add_key(keys_array, key, value, flags) function is used to insert keys into hash keys array;

Wildcard matching

To create a hash that works with wildcards, ngx_hash_combined_t type is used. It includes the hash type described above and has two additional keys arrays: dns_wc_head and dns_wc_tail. The initialization of basic properties is done similarly to a usual hash:

Memory management

Heap

To allocate memory from system heap, the following functions are provided by nginx:

ngx_alloc(size, log) — allocate memory from system heap. This is a wrapper around malloc() with logging support. Allocation error and debugging information is logged to log

ngx_calloc(size, log) — same as ngx_alloc(), but memory is filled with zeroes after allocation

ngx_memalign(alignment, size, log) — allocate aligned memory from system heap. This is a wrapper around posix_memalign() on those platforms which provide it. Otherwise implementation falls back to ngx_alloc() which provides maximum alignment

ngx_free(p) — free allocated memory. This is a wrapper around free()

Pool

Most nginx allocations are done in pools. Memory allocated in an nginx pool is freed automatically when the pool in destroyed. This provides good allocation performance and makes memory control easy.

A pool internally allocates objects in continuous blocks of memory. Once a block is full, a new one is allocated and added to the pool memory block list. When a large allocation is requested which does not fit into a block, such allocation is forwarded to the system allocator and the returned pointer is stored in the pool for further deallocation.

Nginx pool has the type ngx_pool_t. The following operations are supported:

ngx_create_pool(size, log) — create a pool with given block size. The pool object returned is allocated in the pool as well.

Since chain links ngx_chain_t are actively used in nginx, nginx pool provides a way to reuse them. The chain field of ngx_pool_t keeps a list of previously allocated links ready for reuse. For efficient allocation of a chain link in a pool, the function ngx_alloc_chain_link(pool) should be used. This function looks up a free chain link in the pool list and only if it’s empty allocates a new one. To free a link ngx_free_chain(pool, cl) should be called.

Cleanup handlers can be registered in a pool. Cleanup handler is a callback with an argument which is called when pool is destroyed. Pool is usually tied with a specific nginx object (like HTTP request) and destroyed in the end of that object’s lifetime, releasing the object itself. Registering a pool cleanup is a convenient way to release resources, close file descriptors or make final adjustments to shared data, associated with the main object.

A pool cleanup is registered by calling ngx_pool_cleanup_add(pool, size) which returns ngx_pool_cleanup_t pointer to be filled by the caller. The size argument allows allocating context for the cleanup handler.

Shared memory

Shared memory is used by nginx to share common data between processes. Function ngx_shared_memory_add(cf, name, size, tag) adds a new shared memory entry ngx_shm_zone_t to the cycle. The function receives name and size of the zone. Each shared zone must have a unique name. If a shared zone entry with the provided name exists, the old zone entry is reused, if its tag value matches too. Mismatched tag is considered an error. Usually, the address of the module structure is passed as tag, making it possible to reuse shared zones by name within one nginx module.

The shared memory entry structure ngx_shm_zone_t has the following fields:

init — initialization callback, called after shared zone is mapped to actual memory

data — data context, used to pass arbitrary data to the init callback

noreuse — flag, disabling shared zone reuse from the old cycle

tag — shared zone tag

shm — platform-specific object of type ngx_shm_t, having at least the following fields:

Shared zone entries are mapped to actual memory in ngx_init_cycle() after configuration is parsed. On POSIX systems, mmap() syscall is used to create shared anonymous mapping. On Windows, CreateFileMapping()/MapViewOfFileEx() pair is used.

For allocating in shared memory, nginx provides slab pool ngx_slab_pool_t. In each nginx shared zone, a slab pool is automatically created for allocating memory in that zone. The pool is located in the beginning of the shared zone and can be accessed by the expression (ngx_slab_pool_t *) shm_zone->shm.addr. Allocation in shared zone is done by calling one of the functions ngx_slab_alloc(pool, size)/ngx_slab_calloc(pool, size). Memory is freed by calling ngx_slab_free(pool, p).

Slab pool divides all shared zone into pages. Each page is used for allocating objects of the same size. Only the sizes which are powers of 2, and not less than 8, are considered. Other sizes are rounded up to one of these values. For each page, a bitmask is kept, showing which blocks within that page are in use and which are free for allocation. For sizes greater than half-page (usually, 2048 bytes), allocation is done by entire pages.

To protect data in shared memory from concurrent access, mutex is available in the mutex field of ngx_slab_pool_t. The mutex is used by the slab pool while allocating and freeing memory. However, it can be used to protect any other user data structures, allocated in the shared zone. Locking is done by calling ngx_shmtx_lock(&shpool->mutex), unlocking is done by calling ngx_shmtx_unlock(&shpool->mutex).

A log message is formatted in a buffer of size NGX_MAX_ERROR_STR (currently, 2048 bytes) on stack. The message is prepended with error level, process PID, connection id (stored in log->connection) and system error text. For non-debug messages log->handler is called as well to prepend more specific information to the log message. HTTP module sets ngx_http_log_error() function as log handler to log client and server addresses, current action (stored in log->action), client request line, server name etc.

Cycle

Cycle object keeps nginx runtime context, created from a specific configuration. The type of the cycle is ngx_cycle_t. Upon configuration reload a new cycle is created from the new version of nginx configuration. The old cycle is usually deleted after a new one is successfully created. Currently active cycle is held in the ngx_cycle global variable and is inherited by newly started nginx workers.

A cycle is created by the function ngx_init_cycle(). The function receives the old cycle as the argument. It’s used to locate the configuration file and inherit as much resources as possible from the old cycle to keep nginx running smoothly. When nginx starts, a fake cycle called “init cycle” is created and is then replaced by a normal cycle, built from configuration.

Some members of the cycle:

pool — cycle pool. Created for each new cycle

log — cycle log. Initially, this log is inherited from the old cycle. After reading configuration, this member is set to point to new_log

new_log — cycle log, created by the configuration. It’s affected by the root scope error_log directive

connections, connections_n — per-worker array of connections of type ngx_connection_t, created by the event module while initializing each nginx worker. The number of connections is set by the worker_connections directive

free_connections, free_connections_n — the and number of currently available connections. If no connections are available, nginx worker refuses to accept new clients

files, files_n — array for mapping file descriptors to nginx connections. This mapping is used by the event modules, having the NGX_USE_FD_EVENT flag (currently, it’s poll and devpoll)

conf_ctx — array of core module configurations. The configurations are created and filled while reading nginx configuration files

modules, modules_n — array of modules ngx_module_t, both static and dynamic, loaded by current configuration

listening — array of listening objects ngx_listening_t. Listening objects are normally added by the the listen directive of different modules which call the ngx_create_listening() function. Based on listening objects, listen sockets are created by nginx

paths — array of paths ngx_path_t. Paths are added by calling the function ngx_add_path() from modules which are going to operate on certain directories. These directories are created by nginx after reading configuration, if missing. Moreover, two handlers can be added for each path:

path loader — executed only once in 60 seconds after starting or reloading nginx. Normally, reads the directory and stores data in nginx shared memory. The handler is called from a dedicated nginx process “nginx cache loader”

path manager — executed periodically. Normally, removes old files from the directory and reflects these changes in nginx memory. The handler is called from a dedicated nginx process “nginx cache manager”

open_files — list of ngx_open_file_t objects. An open file object is created by calling the function ngx_conf_open_file(). After reading configuration nginx opens all files from the open_files list and stores file descriptors in the fd field of each open file object. The files are opened in append mode and created if missing. The files from this list are reopened by nginx workers upon receiving the reopen signal (usually it’s USR1). In this case the fd fields are changed to new descriptors. The open files are currently used for logging

shared_memory — list of shared memory zones, each added by calling the ngx_shared_memory_add() function. Shared zones are mapped to the same address range in all nginx processes and are used to share common data, for example HTTP cache in-memory tree

Buffer

For input/output operations, nginx provides the buffer type ngx_buf_t. Normally, it’s used to hold data to be written to a destination or read from a source. Buffer can reference data in memory and in file. Technically it’s possible that a buffer references both at the same time. Memory for the buffer is allocated separately and is not related to the buffer structure ngx_buf_t.

The structure ngx_buf_t has the following fields:

start, end — the boundaries of memory block, allocated for the buffer

pos, last — memory buffer boundaries, normally a subrange of start .. end

file_pos, file_last — file buffer boundaries, these are offsets from the beginning of the file

tag — unique value, used to distinguish buffers, created by different nginx module, usually, for the purpose of buffer reuse

file — file object

temporary — flag, meaning that the buffer references writable memory

memory — flag, meaning that the buffer references read-only memory

in_file — flag, meaning that current buffer references data in a file

flush — flag, meaning that all data prior to this buffer should be flushed

recycled — flag, meaning that the buffer can be reused and should be consumed as soon as possible

sync — flag, meaning that the buffer carries no data or special signal like flush or last_buf. Normally, such buffers are considered an error by nginx. This flags allows skipping the error checks

last_buf — flag, meaning that current buffer is the last in output

last_in_chain — flag, meaning that there’s no more data buffers in a (sub)request

shadow — reference to another buffer, related to the current buffer. Usually current buffer uses data from the shadow buffer. Once current buffer is consumed, the shadow buffer should normally also be marked as consumed

last_shadow — flag, meaning that current buffer is the last buffer, referencing a particular shadow buffer

temp_file — flag, meaning that the buffer is in a temporary file

For input and output buffers are linked in chains. Chain is a sequence of chain links ngx_chain_t, defined as follows:

reusable — flag, meaning, that the connection is at the state, when it can be reused

close — flag, meaning, that the connection is being reused and should be closed

An nginx connection can transparently encapsulate SSL layer. In this case the connection ssl field holds a pointer to an ngx_ssl_connection_t structure, keeping all SSL-related data for the connection, including SSL_CTX and SSL. The handlers recv, send, recv_chain, send_chain are set as well to SSL functions.

The number of connections per nginx worker is limited by the worker_connections value. All connection structures are pre-created when a worker starts and stored in the connections field of the cycle object. To reach out for a connection structure, ngx_get_connection(s, log) function is used. The function receives a socket descriptor s which needs to be wrapped in a connection structure.

Since the number of connections per worker is limited, nginx provides a way to grab connections which are currently in use. To enable or disable reuse of a connection, function ngx_reusable_connection(c, reusable) is called. Calling ngx_reusable_connection(c, 1) sets the reuse flag of the connection structure and inserts the connection in the reusable_connections_queue of the cycle. Whenever ngx_get_connection() finds out there are no available connections in the free_connections list of the cycle, it calls ngx_drain_connections() to release a specific number of reusable connections. For each such connection, the close flag is set and its read handler is called which is supposed to free the connection by calling ngx_close_connection(c) and make it available for reuse. To exit the state when a connection can be reused ngx_reusable_connection(c, 0) is called. An example of reusable connections in nginx is HTTP client connections which are marked as reusable until some data is received from the client.

Events

Event

Event object ngx_event_t in nginx provides a way to be notified of a specific event happening.

Some of the fields of the ngx_event_t are:

data — arbitrary event context, used in event handler, usually, a pointer to a connection, tied with the event

handler — callback function to be invoked when the event happens

write — flag, meaning that this is the write event. Used to distinguish between read and write events

cancelable — timer event flag, meaning that the event handler should be called while performing nginx worker graceful shutdown, event though event timeout has not yet expired. The flag provides a way to finalize certain activities, for example, flush log files

posted — flag, meaning that the event is posted to queue

queue — queue node for posting the event to a queue

I/O events

Each connection, received with the ngx_get_connection() call, has two events attached to it: c->read and c->write. These events are used to receive notifications about the socket being ready for reading or writing. All such events operate in Edge-Triggered mode, meaning that they only trigger notifications when the state of the socket changes. For example, doing a partial read on a socket will not make nginx deliver a repeated read notification until more data arrive in the socket. Even when the underlying I/O notification mechanism is essentially Level-Triggered (poll, select etc), nginx will turn the notifications into Edge-Triggered. To make nginx event notifications consistent across all notifications systems on different platforms, it’s required, that the functions ngx_handle_read_event(rev, flags) and ngx_handle_write_event(wev, lowat) are called after handling an I/O socket notification or calling any I/O functions on that socket. Normally, these functions are called once in the end of each read or write event handler.

Timer events

An event can be set to notify a timeout expiration. The function ngx_add_timer(ev, timer) sets a timeout for an event, ngx_del_timer(ev) deletes a previously set timeout. Timeouts currently set for all existing events, are kept in a global timeout Red-Black tree ngx_event_timer_rbtree. The key in that tree has the type ngx_msec_t and is the time in milliseconds since the beginning of January 1, 1970 (modulus ngx_msec_t max value) at which the event should expire. The tree structure provides fast inserting and deleting operations, as well as accessing the nearest timeouts. The latter is used by nginx to find out for how long to wait for I/O events and for expiring timeout events afterwards.

Posted events

An event can be posted which means that its handler will be called at some point later within the current event loop iteration. Posting events is a good practice for simplifying code and escaping stack overflows. Posted events are held in a post queue. The macro ngx_post_event(ev, q) posts the event ev to the post queue q. Macro ngx_delete_posted_event(ev) deletes the event ev from whatever queue it’s currently posted. Normally, events are posted to the ngx_posted_events queue. This queue is processed late in the event loop — after all I/O and timer events are already handled. The function ngx_event_process_posted() is called to process an event queue. This function calls event handlers until the queue is not empty. This means that a posted event handler can post more events to be processed within the current event loop iteration.

Event loop

All nginx processes which do I/O, have an event loop. The only type of process which does not have I/O, is nginx master process which spends most of its time in sigsuspend() call waiting for signals to arrive. Event loop is implemented in ngx_process_events_and_timers() function. This function is called repeatedly until the process exits. It has the following stages:

find nearest timeout by calling ngx_event_find_timer(). This function finds the leftmost timer tree node and returns the number of milliseconds until that node expires

process I/O events by calling a handler, specific to event notification mechanism, chosen by nginx configuration. This handler waits for at least one I/O event to happen, but no longer, than the nearest timeout. For each read or write event which has happened, the ready flag is set and its handler is called. For Linux, normally, the ngx_epoll_process_events() handler is used which calls epoll_wait() to wait for I/O events

expire timers by calling ngx_event_expire_timers(). The timer tree is iterated from the leftmost element to the right until a not yet expired timeout is found. For each expired node the timedout event flag is set, timer_set flag is reset, and the event handler is called

process posted events by calling ngx_event_process_posted(). The function repeatedly removes the first element from the posted events queue and calls its handler until the queue gets empty

All nginx processes handle signals as well. Signal handlers only set global variables which are checked after the ngx_process_events_and_timers() call.

Processes

There are several types of processes in nginx. The type of current process is kept in the ngx_process global variable:

NGX_PROCESS_MASTER — the master process runs the ngx_master_process_cycle() function. Master process does not have any I/O and responds only to signals. It reads configuration, creates cycles, starts and controls child processes

NGX_PROCESS_WORKER — the worker process runs the ngx_worker_process_cycle() function. Worker processes are started by master and handle client connections. They also respond to signals and channel commands, sent from master

NGX_PROCESS_SINGLE — single process is the only type of processes which exist in the master_process off mode. The cycle function for this process is ngx_single_process_cycle(). This process creates cycles and handles client connections

NGX_PROCESS_HELPER — currently, there are two types of helper processes: cache manager and cache loader. Both of them share the same cycle function ngx_cache_manager_process_cycle().

All nginx processes handle the following signals:

NGX_SHUTDOWN_SIGNAL (SIGQUIT) — graceful shutdown. Upon receiving this signal master process sends shutdown signal to all child processes. When no child processes are left, master destroys cycle pool and exits. A worker process which received this signal, closes all listening sockets and waits until timeout tree becomes empty, then destroys cycle pool and exits. A cache manager process exits right after receiving this signal. The variable ngx_quit is set to one after receiving this signal and immediately reset after being processed. The variable ngx_exiting is set to one when worker process is in shutdown state

NGX_TERMINATE_SIGNAL (SIGTERM) - terminate. Upon receiving this signal master process sends terminate signal to all child processes. If child processes do not exit in 1 second, they are killed with the SIGKILL signal. When no child processes are left, master process destroys cycle pool and exits. A worker or cache manager process which received this signal destroys cycle pool and exits. The variable ngx_terminate is set to one after receiving this signal

NGX_RECONFIGURE_SIGNAL (SIGHUP) - reconfigure. Upon receiving this signal master process creates a new cycle from configuration file. If the new cycle was created successfully, the old cycle is deleted and new child processes are started. Meanwhile, the old processes receive the shutdown signal. In single-process mode, nginx creates a new cycle as well, but keeps the old one until all clients, tied to the old cycle, are gone. Worker and helper processes ignore this signal

NGX_CHANGEBIN_SIGNAL (SIGUSR2) — change nginx binary. Master process starts a new nginx binary and passes there a list of all listen sockets. The list is passed in the environment variable “NGINX” in text format, where descriptor numbers separated with semicolons. A new nginx instance reads that variable and adds the sockets to its init cycle. Other processes ignore this signal

While all nginx worker processes are able to receive and properly handle POSIX signals, master process normally does not pass any signals to workers and helpers with the standard kill() syscall. Instead, nginx uses inter-process channels which allow sending messages between all nginx processes. Currently, however, messages are only sent from master to its children. Those messages carry the same signals. The channels are socketpairs with their ends in different processes.

When running nginx binary, several values can be specified next to -s parameter. Those values are stop, quit, reopen, reload. They are converted to signals NGX_TERMINATE_SIGNAL, NGX_SHUTDOWN_SIGNAL, NGX_REOPEN_SIGNAL and NGX_RECONFIGURE_SIGNAL and sent to the nginx master process, whose pid is read from nginx pid file.

Threads

It is possible to offload tasks that would otherwise block nginx worker process into a separate thread. For example, nginx may be configured to use threads to perform file I/O. Another example is using a library that doesn’t have asynchronous interface and thus cannot be normally used with nginx. Keep in mind that threads interface is a helper for existing asynchronous approach in processing client connections, and by no means a replacement.

To deal with synchronization the following wrappers over pthreads primitives are available:

Instead of creating a new thread for each task, nginx implements a thread_pool strategy. Multiple thread pools may be configured intended for different purposes (for example, performing I/O on different sets of disks). Each thread pool is created on start and contains a limited number of threads that process a queue of tasks. When a task is completed, a predefined completion handler is called.

At configuration time, a module willing to use threads has to obtain a reference to thread pool by calling ngx_thread_pool_add(cf, name) which will either create a new thread pool with given name or return a reference to an existing one if a pool with such name already exists.

At runtime, the ngx_thread_task_post(tp, task) function is used to add a task into a queue of a thread pool tp. The ngx_thread_task_t structure contains all necessary to execute user function in thread, pass parameters and setup completion handler:

Modules

Adding new modules

The standalone nginx module resides in a separate directory that contains at least two files: config and a file with the module source. The first file contains all information needed for nginx to integrate the module, for example:

ngx_module_name — the name of the module. A whitespace separated values list is accepted and may be used to build multiple modules from a single set of source files. The first name indicates the name of the output binary for a dynamic module. The names in this list should match the names used in the module.

ngx_addon_name — supplies the name of the module in the console output text of the configure script.

ngx_module_srcs — a whitespace separated list of source files used to compile the module. The $ngx_addon_dir variable can be used as a placeholder for the path of the module source.

ngx_module_incs — include paths required to build the module

ngx_module_deps — a list of module’s header files.

ngx_module_libs — a list of libraries to link with the module. For example, libpthread would be linked using ngx_module_libs=-lpthread. The following macros can be used to link against the same libraries as nginx: LIBXSLT, LIBGD, GEOIP, PCRE, OPENSSL, MD5, SHA1, ZLIB, and PERL

ngx_module_link — set by the build system to DYNAMIC for a dynamic module or ADDON for a static module and used to perform different actions depending on linking type.
ngx_module_order — sets the load order for the module which is useful for HTTP_FILTER and HTTP_AUX_FILTER module types. The order is stored in a reverse list.

The ngx_http_copy_filter_module is near the bottom of the list so is one of the first to be executed. This reads the data for other filters. Near the top of the list is ngx_http_write_filter_module which writes the data out and is one of the last to be executed.

The format for this option is typically the current module’s name followed by a whitespace separated list of modules to insert before, and therefore execute after. The module will be inserted before the last module in the list that is found to be currently loaded.

By default for filter modules this is set to “ngx_http_copy_filter” which will insert the module before the copy filter in the list and therefore will execute after the copy filter. For other module types the default is empty.

A module can be added to nginx by means of the configure script using –add-module=/path/to/module for static compilation and –add-dynamic-module=/path/to/module for dynamic compilation.

Core modules

Modules are building blocks of nginx, and most of its functionality is implemented as modules. The module source file must contain a global variable of ngx_module_t type which is defined as follows:

The omitted private part includes module version, signature and is filled using the predefined macro NGX_MODULE_V1.

Each module keeps its private data in the ctx field, recognizes specific configuration directives, specified in the commands array, and may be invoked at certain stages of nginx lifecycle. The module lifecycle consists of the following events:

Configuration directive handlers are called as they appear in configuration files in the context of the master process

The init_module handler is called in the context of the master process after the configuration is parsed successfully

The master process creates worker process(es) and init_process handler is called in each of them

When a worker process receives the shutdown command from master, it invokes the exit_process handler

The master process calls the exit_master handler before exiting.

init_module handler may be called multiple times in the master process if the configuration reload is requested.

The init_master, init_thread and exit_thread handlers are not implemented at the moment; Threads in nginx are only used as supplementary I/O facility with its own API and init_master handler looks unnecessary.

The module type defines what exactly is stored in the ctx field. There are several types of modules:

NGX_CORE_MODULE

NGX_EVENT_MODULE

NGX_HTTP_MODULE

NGX_MAIL_MODULE

NGX_STREAM_MODULE

The NGX_CORE_MODULE is the most basic and thus the most generic and most low-level type of module. Other module types are implemented on top of it and provide more convenient way to deal with corresponding problem domains, like handling events or http requests.

where the name is a string with a module name for convenience, create_conf and init_conf are pointers to functions that create and initialize module configuration correspondingly. For core modules, nginx will call create_conf before parsing a new configuration and init_conf after all configuration was parsed successfully. The typical create_conf function allocates memory for the configuration and sets default values. The init_conf deals with known configuration and thus may perform sanity checks and complete initialization.

Configuration directives

The ngx_command_t describes single configuration directive. Each module, supporting configuration, provides an array of such specifications that describe how to process arguments and what handlers to call:

The array should be terminated by a special value “ngx_null_command”. The name is the literal name of a directive, as it appears in configuration file, for example “worker_processes” or “listen”. The type is a bitfield that controls number of arguments, command type and other properties using corresponding flags. Arguments flags:

NGX_CONF_NOARGS — directive without arguments

NGX_CONF_1MORE — one required argument

NGX_CONF_2MORE — two required arguments

NGX_CONF_TAKE1..7 — exactly 1..7 arguments

NGX_CONF_TAKE12, 13, 23, 123, 1234 — one or two arguments, or other combinations

Directive types:

NGX_CONF_BLOCK — the directive is a block, i.e. it may contain other directives in curly braces, or even implement its own parser to handle contents inside.

NGX_CONF_FLAG — the directive value is a flag, a boolean value represented by “on” or “off” strings.

Context of a directive defines where in the configuration it may appear and how to access module context to store corresponding values:

NGX_MAIN_CONF — top level configuration

NGX_HTTP_MAIN_CONF — in the http block

NGX_HTTP_SRV_CONF — in the http server block

NGX_HTTP_LOC_CONF — in the http location

NGX_HTTP_UPS_CONF — in the http upstream block

NGX_HTTP_SIF_CONF — in the http server “if”

NGX_HTTP_LIF_CONF — in the http location “if”

NGX_HTTP_LMT_CONF — in the http “limit_except”

NGX_STREAM_MAIN_CONF — in the stream block

NGX_STREAM_SRV_CONF — in the stream server block

NGX_STREAM_UPS_CONF — in the stream upstream block

NGX_MAIL_MAIN_CONF — in the the mail block

NGX_MAIL_SRV_CONF — in the mail server block

NGX_EVENT_CONF — in the event block

NGX_DIRECT_CONF — used by modules that don’t create a hierarchy of contexts and store module configuration directly in ctx

The configuration parser uses this flags to throw an error in case of a misplaced directive and calls directive handlers supplied with a proper configuration pointer, so that same directives in different locations could store their values in distinct places.

The set field defines a handler that processes a directive and stores parsed values into corresponding configuration. Nginx offers a convenient set of functions that perform common conversions:

ngx_conf_set_bitmask_slot — arguments are converted to ngx_uint_t value and OR’ed with the resulting value, forming a bitmask. The null-terminated array of ngx_conf_bitmask_t passed in the post field defines acceptable strings and corresponding mask values

set_path_slot — converts arguments to ngx_path_t type and performs all required initializations. See the proxy_temp_path directive description for details

The conf field defines which context is used to store the value of the directive, or zero if contexts are not used. Only simple core modules use configuration without context and set NGX_DIRECT_CONF flag. In real life, such modules like http or stream require more sophisticated configuration that can be applied per-server or per-location, or even more precisely, in the context of the “if” directive or some limit. In this modules, configuration structure is more complex. Please refer to corresponding modules description to understand how they manage their configuration.

NGX_HTTP_MAIN_CONF_OFFSET — http block configuration

NGX_HTTP_SRV_CONF_OFFSET — http server configuration

NGX_HTTP_LOC_CONF_OFFSET — http location configuration

NGX_STREAM_MAIN_CONF_OFFSET — stream block configuration

NGX_STREAM_SRV_CONF_OFFSET — stream server configuration

NGX_MAIL_MAIN_CONF_OFFSET — mail block configuration

NGX_MAIL_SRV_CONF_OFFSET — mail server configuration

The offset defines an offset of a field in a module configuration structure that holds values of this particular directive. The typical use is to employ offsetof() macro.

he post is a twofold field: it may be used to define a handler to be called after main handler completed or to pass additional data to the main handler. In the first case, ngx_conf_post_t structure needs to be initialized with a pointer to handler, for example:

The post argument is the ngx_conf_post_t object itself, and the data is a pointer to value, converted from arguments by the main handler with the appropriate type.

HTTP

Connection

Each client HTTP connection runs through the following stages:

ngx_event_accept() accepts a client TCP connection. This handler is called in response to a read notification on a listen socket. A new ngx_connecton_t object is created at this stage. The object wraps the newly accepted client socket. Each nginx listener provides a handler to pass the new connection object to. For HTTP connections it’s ngx_http_init_connection(c)

ngx_http_init_connection() performs early initialization of an HTTP connection. At this stage an ngx_http_connection_t object is created for the connection and its reference is stored in connection’s data field. Later it will be substituted with an HTTP request object. PROXY protocol parser and SSL handshake are started at this stage as well

ngx_http_wait_request_handler() is a read event handler, that is called when data is available in the client socket. At this stage an HTTP request object ngx_http_request_t is created and set to connection’s data field

ngx_http_process_request_line() is a read event handler, which reads client request line. The handler is set by ngx_http_wait_request_handler(). Reading is done into connection’s buffer. The size of the buffer is initially set by the directive client_header_buffer_size. The entire client header is supposed to fit the buffer. If the initial size is not enough, a bigger buffer is allocated, whose size is set by the large_client_header_buffers directive

ngx_http_process_request_headers() is a read event handler, which is set after ngx_http_process_request_line() to read client request header

ngx_http_core_run_phases() is called when the request header is completely read and parsed. This function runs request phases from NGX_HTTP_POST_READ_PHASE to NGX_HTTP_CONTENT_PHASE. The last phase is supposed to generate response and pass it along the filter chain. The response is not necessarily sent to the client at this phase. It may remain buffered and will be sent at the finalization stage

ngx_http_finalize_request() is usually called when the request has generated all the output or produced an error. In the latter case an appropriate error page is looked up and used as the response. If the response is not completely sent to the client by this point, an HTTP writer ngx_http_writer() is activated to finish sending outstanding data

ngx_http_finalize_connection() is called when the response is completely sent to the client and the request can be destroyed. If client connection keepalive feature is enabled, ngx_http_set_keepalive() is called, which destroys current request and waits for the next request on the connection. Otherwise, ngx_http_close_request() destroys both the request and the connection

Request

For each client HTTP request the ngx_http_request_t object is created. Some of the fields of this object:

connection — pointer to a ngx_connection_t client connection object. Several requests may reference the same connection object at the same time - one main request and its subrequests. After a request is deleted, a new request may be created on the same connection.

Note that for HTTP connections ngx_connection_t’s data field points back to the request. Such request is called active, as opposed to the other requests tied with the connection. Active request is used to handle client connection events and is allowed to output its response to the client. Normally, each request becomes active at some point to be able to send its output

ctx — array of HTTP module contexts. Each module of type NGX_HTTP_MODULE can store any value (normally, a pointer to a structure) in the request. The value is stored in the ctx array at the module’s ctx_index position. The following macros provide a convenient way to get and set request contexts:

read_event_handler, write_event_handler - read and write event handlers for the request. Normally, an HTTP connection has ngx_http_request_handler() set as both read and write event handlers. This function calls read_event_handler and write_event_handler handlers of the currently active request

cache — request cache object for caching upstream response

upstream — request upstream object for proxying

pool — request pool. This pool is destroyed when the request is deleted. The request object itself is allocated in this pool. For allocations which should be available throughout the client connection’s lifetime, ngx_connection_t’s pool should be used instead

header_in — buffer where client HTTP request header in read

headers_in, headers_out — input and output HTTP headers objects. Both objects contain the headers field of type ngx_list_t keeping the raw list of headers. In addition to that, specific headers are available for getting and setting as separate fields, for example content_length_n, status etc

request_body — client request body object

start_sec, start_msec — time point when the request was created. Used for tracking request duration

http_protocol, http_version, http_major, http_minor - client HTTP protocol version in its original textual form (“HTTP/1.0”, “HTTP/1.1” etc), numeric form (NGX_HTTP_VERSION_10, NGX_HTTP_VERSION_11 etc) and separate major and minor versions

request_line, unparsed_uri — client original request line and URI

uri, args, exten — current request URI, arguments and file extention. The URI value here might differ from the original URI sent by the client due to normalization. Throughout request processing, these value can change while performing internal redirects

main — pointer to a main request object. This object is created to process client HTTP request, as opposed to subrequests, created to perform a specific sub-task within the main request

parent — pointer to a parent request of a subrequest

postponed — list of output buffers and subrequests in the order they are sent and created. The list is used by the postpone filter to provide consistent request output, when parts of it are created by subrequests

post_subrequest — pointer to a handler with context to be called when a subrequest gets finalized. Unused for main requests

posted_requests — list of requests to be started or resumed. Starting or resuming is done by calling the request’s write_event_handler. Normally, this handler holds the request main function, which at first runs request phases and then produces the output.

A request is usually posted by the ngx_http_post_request(r, NULL) call. It is always posted to the main request posted_requests list. The function ngx_http_run_posted_requests(c) runs all requests, posted in the main request of the passed connection’s active request. This function should be called in all event handlers, which can lead to new posted requests. Normally, it’s called always after invoking a request’s read or write handler

phase_handler — index of current request phase

ncaptures, captures, captures_data — regex captures produced by the last regex match of the request. While processing a request, there’s a number of places where a regex match can happen: map lookup, server lookup by SNI or HTTP Host, rewrite, proxy_redirect etc. Captures produced by a lookup are stored in the above mentioned fields. The field ncaptures holds the number of captures, captures holds captures boundaries, captures_data holds a string, against which the regex was matched and which should be used to extract captures. After each new regex match request captures are reset to hold new values

count — request reference counter. The field only makes sense for the main request. Increasing the counter is done by simple r->main->count++. To decrease the counter ngx_http_finalize_request(r, rc) should be called. Creation of a subrequest or running request body read process increase the counter

subrequests — current subrequest nesting level. Each subrequest gets the nesting level of its parent decreased by one. Once the value reaches zero an error is generated. The value for the main request is defined by the NGX_HTTP_MAX_SUBREQUESTS constant

uri_changes — number of URI changes left for the request. The total number of times a request can change its URI is limited by the NGX_HTTP_MAX_URI_CHANGES constant. With each change the value is decreased until it reaches zero. In the latter case an error is generated. The actions considered as URI changes are rewrites and internal redirects to normal or named locations

blocked — counter of blocks held on the request. While this value is non-zero, request cannot be terminated. Currently, this value is increased by pending AIO operations (POSIX AIO and thread operations) and active cache lock

buffered — bitmask showing which modules have buffered the output produced by the request. A number of filters can buffer output, for example sub_filter can buffer data due to a partial string match, copy filter can buffer data because of the lack of free output_buffers etc. As long as this value is non-zero, request is not finalized, expecting the flush

header_only — flag showing that output does not require body. For example, this flag is used by HTTP HEAD requests

keepalive — flag showing if client connection keepalive is supported. The value is inferred from HTTP version and “Connection” header value

header_sent — flag showing that output header has already been sent by the request

internal — flag showing that current request is internal. To enter the internal state, a request should pass through an internal redirect or be a subrequest. Internal requests are allowed to enter internal locations

allow_ranges — flag showing that partial response can be sent to client, if requested by the HTTP Range header

subrequest_ranges — flag showing that a partial response is allowed to be sent while processing a subrequest

single_range — flag showing that only a single continuous range of output data can be sent to the client. This flag is usually set when sending a stream of data, for example from a proxied server, and the entire response is not available at once

main_filter_need_in_memory, filter_need_in_memory — flags showing that the output should be produced in memory buffers but not in files. This is a signal to the copy filter to read data from file buffers even if sendfile is enabled. The difference between these two flags is the location of filter modules which set them. Filters called before the postpone filter in filter chain, set filter_need_in_memory requesting that only the current request output should come in memory buffers. Filters called later in filter chain set main_filter_need_in_memory requiring that both the main request and all the subrequest read files in memory while sending output

filter_need_temporary — flag showing that the request output should be produced in temporary buffers, but not in readonly memory buffers or file buffers. This is used by filters which may change output directly in the buffers, where it’s sent

Configuration

Each HTTP module may have three types of configuration:

Main configuration. This configuration applies to the entire nginx http{} block. This is global configuration. It stores global settings for a module

Server configuration. This configuraion applies to a single nginx server{}. It stores server-specific settings for a module

Location configuration. This configuraion applies to a single location{}, if{} or limit_except() block. This configuration stores settings specific to a location

Configuration structures are created at nginx configuration stage by calling functions, which allocate these structures, initialize them and merge. The following example shows how to create a simple module location configuration. The configuration has one setting foo of unsiged integer type.

As seen in the example, ngx_http_foo_create_loc_conf() function creates a new configuration structure and ngx_http_foo_merge_loc_conf() merges a configuration with another configuration from a higher level. In fact, server and location configuration do not only exist at server and location levels, but also created for all the levels above. Specifically, a server configuration is created at the main level as well and location configurations are created for main, server and location levels. These configurations make it possible to specify server and location-specific settings at any level of nginx configuration file. Eventually configurations are merged down. To indicate a missing setting and ignore it while merging, nginx provides a number of macros like NGX_CONF_UNSET and NGX_CONF_UNSET_UINT. Standard nginx merge macros like ngx_conf_merge_value() and ngx_conf_merge_uint_value() provide a convenient way to merge a setting and set the default value if none of configurations provided an explicit value. For complete list of macros for different types see src/core/ngx_conf_file.h.

To access configuration of any HTTP module at configuration time, the following macros are available. They receive ngx_conf_t reference as the first argument.

ngx_http_conf_get_module_main_conf(cf, module)

ngx_http_conf_get_module_srv_conf(cf, module)

ngx_http_conf_get_module_loc_conf(cf, module)

The following example gets a pointer to a location configuration of standard nginx core module ngx_http_core_module and changes location content handler kept in the handler field of the structure.

In runtime the following macros are available to get configurations of HTTP modules.

ngx_http_get_module_main_conf(r, module)

ngx_http_get_module_srv_conf(r, module)

ngx_http_get_module_loc_conf(r, module)

These macros receive a reference to an HTTP request ngx_http_request_t. Main configuration of a request never changes. Server configuration may change from a default one after choosing a virtual server for a request. Request location configuration may change multiple times as a result of a rewrite or internal redirect. The following example shows how to access HTTP configuration in runtime.

Phases

Each HTTP request passes through a list of HTTP phases. Each phase is specialized in a particular type of processing. Most phases allow installing handlers. The phase handlers are called successively once the request reaches the phase. Many standard nginx modules install their phase handlers as a way to get called at a specific request processing stage. Following is the list of nginx HTTP phases.

NGX_HTTP_POST_READ_PHASE is the earliest phase. The ngx_http_realip_module installs its handler at this phase. This allows to substitute client address before any other module is invoked

NGX_HTTP_SERVER_REWRITE_PHASE is used to run rewrite script, defined at the server level, that is out of any location block. The ngx_http_rewrite_module installs its handler at this phase

NGX_HTTP_FIND_CONFIG_PHASE — a special phase used to choose a location based on request URI. This phase does not allow installing any handlers. It only performs the default action of choosing a location. Before this phase, the server default location is assigned to the request. Any module requesting a location configuration, will receive the default server location configuration. After this phase a new location is assigned to the request

NGX_HTTP_REWRITE_PHASE — same as NGX_HTTP_SERVER_REWRITE_PHASE, but for a new location, chosen at the prevous phase

NGX_HTTP_POST_REWRITE_PHASE — a special phase, used to redirect the request to a new location, if the URI was changed during rewrite. The redirect is done by going back to NGX_HTTP_FIND_CONFIG_PHASE. No handlers are allowed at this phase

NGX_HTTP_PREACCESS_PHASE — a common phase for different types of handlers, not associated with access check. Standard nginx modules ngx_http_limit_conn_module and ngx_http_limit_req_module register their handlers at this phase

NGX_HTTP_ACCESS_PHASE — used to check access permissions for the request. Standard nginx modules such as ngx_http_access_module and ngx_http_auth_basic_module register their handlers at this phase. If configured so by the satisfy directive, only one of access phase handlers may allow access to the request in order to confinue processing

NGX_HTTP_POST_ACCESS_PHASE — a special phase for the satisfy any case. If some access phase handlers denied the access and none of them allowed, the request is finalized. No handlers are supported at this phase

NGX_HTTP_TRY_FILES_PHASE — a special phase, for the try_files feature. No handlers are allowed at this phase

NGX_HTTP_CONTENT_PHASE — a phase, at which the response is supposed to be generated. Multiple nginx standard modules register their handers at this phase, for example ngx_http_index_module or ngx_http_static_module. All these handlers are called sequentially until one of them finally produces the output. It’s also possible to set content handlers on a per-location basis. If the ngx_http_core_module’s location configuration has handler set, this handler is called as the content handler and content phase handlers are ignored

NGX_HTTP_LOG_PHASE is used to perform request logging. Currently, only the ngx_http_log_module registers its handler at this stage for access logging. Log phase handlers are called at the very end of request processing, right before freeing the request

NGX_DECLINED — proceed to the next handler of the current phase. If current handler is the last in current phase, move to the next phase

NGX_AGAIN, NGX_DONE — suspend phase handling until some future event. This can be for example asynchronous I/O operation or just a delay. It is supposed, that phase handling will be resumed later by calling ngx_http_core_run_phases()

Any other value returned by the phase handler is treated as a request finalization code, in particular, HTTP response code. The request is finalized with the code provided

Some phases treat return codes in a slightly different way. At content phase, any return code other that NGX_DECLINED is considered a finalization code. As for the location content handlers, any return from them is considered a finalization code. At access phase, in satisfy any mode, returning a code other than NGX_OK, NGX_DECLINED, NGX_AGAIN, NGX_DONE is considered a denial. If none of future access handlers allow access or deny with a new code, the denial code will become the finalization code.

Variables

Accessing existing variables

Variables may be referenced using index (this is the most common method) or names (see below in the section about creating variables). Index is created at configuration stage, when a variable is added to configuration. The variable index can be obtained using ngx_http_get_variable_index():

Here, the cf is a pointer to nginx configuration and the name points to a string with the variable name. The function returns NGX_ERROR on error or valid index otherwise, which is typically stored somewhere in a module configuration for future use.

All HTTP variables are evaluated in the context of HTTP request and results are specific to and cached in HTTP request. All functions that evaluate variables return ngx_http_variable_value_t type, representing the variable value:

not_found — variable was not found and thus the data and len fields are irrelevant; this may happen, for example, with such variables as $arg_foo when a corresponding argument was not passed in a request

no_cacheable — do not cache result

escape — used internally by the logging module to mark values that require escaping on output

The ngx_http_get_flushed_variable() and ngx_http_get_indexed_variable() functions are used to obtain the variable value. They have the same interface - accepting a HTTP request r as a context for evaluating the variable and an index, identifying it. Example of typical usage:

ngx_http_variable_value_t *v;
v = ngx_http_get_flushed_variable(r, index);
if (v == NULL || v->not_found) {
/* we failed to get value or there is no such variable, handle it */
return NGX_ERROR;
}
/* some meaningful value is found */

The difference between functions is that the ngx_http_get_indexed_variable() returns cached value and ngx_http_get_flushed_variable() flushes cache for non-cacheable variables.

There are cases when it is required to deal with variables which names are not known at configuration time and thus they cannot be accessed using indexes, for example in modules like SSI or Perl. The ngx_http_get_variable(r, name, key) function may be used in such cases. It searches for the variable with a given name and its hash key.

Creating variables

To create a variable ngx_http_add_variable() function is used. It takes configuration (where variable is registered), variable name and flags that control its behaviour:

NGX_HTTP_VAR_CHANGEABLE — allows redefining the variable; If another module will define a variable with such name, no conflict will happen. For example, this allows user to override variables using the set directive.

NGX_HTTP_VAR_NOCACHEABLE — disables caching, is useful for such variables as $time_local

NGX_HTTP_VAR_NOHASH — indicates that this variable is only accessible by index, not by name. This is a small optimization which may be used when it is known that the variable is not needed in modules like SSI or Perl.

NGX_HTTP_VAR_PREFIX — the name of this variable is a prefix. A handler must implement additional logic to obtain value of specific variable. For example, all “arg_” variables are processed by the same handler which performs lookup in request arguments and returns value of specific argument.

The function returns NULL in case of error or a pointer to ngx_http_variable_t:

It returns NGX_ERROR in case of internal error (for example, failed memory allocation) or NGX_OK otherwise. The status of variable evaluation may be understood by inspecting flags of the ngx_http_variable_value_t (see description above).

The set handler allows setting the property referred by the variable. For example, the $limit_rate variable set handler modifies the request’s limit_rate field:

The zero flag is usable when results are to be passed to libraries that require zero-terminated strings, and prefixes are handy when dealing with filenames.

Upon successful compilation, cv.lengths may be inspected to get information about the presence of variables in the expression. The NULL value means that the expression contained static text only, and there is no need in storing it as a complex value, so a simple string can be used.

The ngx_http_set_complex_value_slot() is a convenient function used to initialize complex value completely right in the directive declaration.

At runtime, a complex value may be calculated using the ngx_http_complex_value() function:

Given the request r and previously compiled value cv the function will evaluate expression and put result into res.

Request redirection

An HTTP request is always connected to a location via the loc_conf field of the ngx_http_request_t structure. This means that at any point the location configuration of any module can be retrieved from the request by calling ngx_http_get_module_loc_conf(r, module). Request location may be changed several times throughout its lifetime. Initially, a default server location of the default server is assigned to a request. Once a request switches to a different server (chosen by the HTTP “Host” header or SSL SNI extension), the request switches to the default location of that server as well. The next change of the location takes place at the NGX_HTTP_FIND_CONFIG_PHASE request phase. At this phase a location is chosen by request URI among all non-named locations configured for the server. The ngx_http_rewrite_module may change the request URI at the NGX_HTTP_REWRITE_PHASE request phase as a result of rewrite and return to the NGX_HTTP_FIND_CONFIG_PHASE phase for choosing a new location based on the new URI.

It is also possible to redirect a request to a new location at any point by calling one of the functions ngx_http_internal_redirect(r, uri, args) or ngx_http_named_location(r, name).

The function ngx_http_internal_redirect(r, uri, args) changes the request URI and returns the request to the NGX_HTTP_SERVER_REWRITE_PHASE phase. The request proceeds with a server default location. Later at NGX_HTTP_FIND_CONFIG_PHASE a new location is chosen based on the new request URI.

The following example performs an internal redirect with the new request arguments.

The function ngx_http_named_location(r, name) redirects a request to a named location. The name of the location is passed as the argument. The location is looked up among all named locations of the current server, after which the requests switches to the NGX_HTTP_REWRITE_PHASE phase.

Both functions ngx_http_internal_redirect(r, uri, args) and ngx_http_named_location(r, name) may be called when a request already has some contexts saved in its ctx field by nginx modules. These contexts could become inconsistent with the new location configuration. To prevent inconsistency, all request contexts are erased by both redirect functions.

Redirected and rewritten requests become internal and may access the internal locations. Internal requests have the internal flag set.

Subrequests

Subrequests are primarily used to include output of one request into another, possibly mixed with other data. A subrequest looks like a normal request, but shares some data with its parent. Particularly, all fields related to client input are shared since a subrequest does not receive any other input from client. The request field parent for a subrequest keeps a link to its parent request and is NULL for the main request. The field main keeps a link to the main request in a group of requests.

A subrequest starts with NGX_HTTP_SERVER_REWRITE_PHASE phase. It passes through the same phases as a normal request and is assigned a location based on its own URI.

Subrequest output header is always ignored. Subrequest output body is placed by the ngx_http_postpone_filter into the right position in relation to other data produced by the parent request.

Subrequests are related to the concept of active requests. A request r is considered active if c->data == r, where c is the client connection object. At any point, only the active request in a request group is allowed to output its buffers to the client. A non-active request can still send its data to the filter chain, but they will not pass beyond the ngx_http_postpone_filter and will remain buffered by that filter until the request becomes active. Here are some rules of request activation:

Initially, the main request is active

The first subrequest of an active request becomes active right after creation

The ngx_http_postpone_filter activates the next request in active request’s subrequest list, once all data prior to that request are sent

When a request is finalized, its parent is activated

A subrequest is created by calling the function ngx_http_subrequest(r, uri, args, psr, ps, flags), where r is the parent request, uri and args are URI and arguments of the subrequest, psr is the output parameter, receiving the newly created subrequest reference, ps is a callback object for notifying the parent request that the subrequest is being finalized, flags is subrequest creation flags bitmask. The following flags are available:

NGX_HTTP_SUBREQUEST_IN_MEMORY - subrequest output should not be sent to the client, but rather stored in memory. This only works for proxying subrequests. After subrequest finalization its output is available in r->upstream->buffer buffer of type ngx_buf_t

NGX_HTTP_SUBREQUEST_WAITED - the subrequest done flag is set even if it is finalized being non-active. This subrequest flag is used by the SSI filter

NGX_HTTP_SUBREQUEST_CLONE - the subrequest is created as a clone of its parent. It is started at the same location and proceeds from the same phase as the parent request

Subrequests are normally created in a body filter. In this case subrequest output can be treated as any other explicit request output. This means that eventually the output of a subrequest is sent to the client after all explicit buffers passed prior to subrequest creation and before any buffers passed later. This ordering is preserved even for large hierarchies of subrequests. The following example inserts a subrequest output after all request data buffers, but before the final buffer with the last_buf flag.

A subrequest may also be created for other purposes than data output. For example, the ngx_http_auth_request_module creates a subrequest at NGX_HTTP_ACCESS_PHASE phase. To disable any output at this point, the subrequest header_only flag is set. This prevents subrequest body from being sent to the client. Its header is ignored anyway. The result of the subrequest can be analyzed in the callback handler.

Request finalization

An HTTP request is finalized by calling the function ngx_http_finalize_request(r, rc). It is usually finalized by the content handler after sending all output buffers to the filter chain. At this point the output may not be completely sent to the client, but remain buffered somewhere along the filter chain. If it is, the ngx_http_finalize_request(r, rc) function will automatically install a special handler ngx_http_writer(r) to finish sending the output. A request is also finalized in case of an error or if a standard HTTP response code needs to be returned to the client.

The function ngx_http_finalize_request(r, rc) expects the following rc values:

NGX_DONE - fast finalization. Decrement request count and destroy the request if it reaches zero. The client connection may still be used for more requests after that

NGX_ERROR, NGX_HTTP_REQUEST_TIME_OUT (408), NGX_HTTP_CLIENT_CLOSED_REQUEST (499) - error finalization. Terminate the request as soon as possible and close the client connection.

NGX_HTTP_CREATED (201), NGX_HTTP_NO_CONTENT (204), codes greater than or equal to NGX_HTTP_SPECIAL_RESPONSE (300) - special response finalization. For these values nginx either sends a default code response page to the client or performs the internal redirect to an error_page location if it’s configured for the code

Other codes are considered success finalization codes and may activate the request writer to finish sending the response body. Once body is completely sent, request count is decremented. If it reaches zero, the request is destroyed, but the client connection may still be used for other requests. If count is positive, there are unfinished activities within the request, which will be finalized at a later point.

Request body

For dealing with client request body, nginx provides the following functions: ngx_http_read_client_request_body(r, post_handler) and ngx_http_discard_request_body(r). The first function reads the request body and makes it available via the request_body request field. The second function instructs nginx to discard (read and ignore) the request body. One of these functions must be called for every request. Normally, it is done in the content handler.

Reading or discarding client request body from a subrequest is not allowed. It should always be done in the main request. When a subrequest is created, it inherits the parent request_body object which can be used by the subrequest if the main request has previously read the request body.

The function ngx_http_read_client_request_body(r, post_handler) starts the process of reading the request body. When the body is completely read, the post_handler callback is called to continue processing the request. If request body is missing or already read, the callback is called immediately. The function ngx_http_read_client_request_body(r, post_handler) allocates the request_body request field of type ngx_http_request_body_t. The field bufs of this object keeps the result as a buffer chain. The body can be saved in memory buffers or file buffers, if client_body_buffer_size is not enough to fit the entire body in memory.

The following fields of the request affect the way request body is read:

request_body_in_single_buf - read body to a single memory buffer

request_body_in_file_only - always read body to a file, even if fits the memory buffer

request_body_in_persistent_file - do not unlink the file right after creation. Such a file can be moved to another directory

request_body_in_clean_file - unlink the file the when the request is finalized. This can be useful when a file was supposed to be moved to another directory but eventually was not moved for some reason

request_body_file_group_access - enable file group access. By default a file is created with 0600 access mask. When the flag is set, 0660 access mask is used

request_body_file_log_level - log file errors with this log level

request_body_no_buffering - read request body without buffering

When the request_body_no_buffering flag is set, the unbuffered mode of reading the request body is enabled. In this mode, after calling ngx_http_read_client_request_body(), the bufs chain may keep only a part of the body. To read the next part, the ngx_http_read_unbuffered_request_body(r) function should be called. The return value of NGX_AGAIN and the request flag reading_body indicate that more data is available. If bufs is NULL after calling this function, there is nothing to read at the moment. The request callback read_event_handler will be called when the next part of request body is available.

Response

An HTTP response in nginx is produced by sending the response header followed by the optional response body. Both header and body are passed through a chain of filters and eventually get written to the client socket. An nginx module can install its handler into the header or body filter chain and process the output coming from the previous handler.

Response header

Output header is sent by the function ngx_http_send_header(r). Prior to calling this function, r->headers_out should contain all the data required to produce the HTTP response header. It’s always required to set the status field of r->headers_out. If the response status suggests that a response body follows the header, content_length_n can be set as well. The default value for this field is -1, which means that the body size is unknown. In this case, chunked transfer encoding is used. To output an arbitrary header, headers list should be appended.

Header filters

The ngx_http_send_header(r) function invokes the header filter chain by calling the top header filter handler ngx_http_top_header_filter. It’s assumed that every header handler calls the next handler in chain until the final handler ngx_http_header_filter(r) is called. The final header handler constructs the HTTP response based on r->headers_out and passes it to the ngx_http_writer_filter for output.

To add a handler to the header filter chain, one should store its address in ngx_http_top_header_filter global variable at configuration time. The previous handler address is normally stored in a module’s static variable and is called by the newly added handler before exiting.

The following is an example header filter module, adding the HTTP header “X-Foo: foo” to every output with the status 200.

Response body

Response body is sent by calling the function ngx_http_output_filter(r, cl). The function can be called multiple times. Each time it sends a part of the response body passed as a buffer chain. The last body buffer should have the last_buf flag set.

The following example produces a complete HTTP output with “foo” as its body. In order for the example to work not only as a main request but as a subrequest as well, the last_in_chain flag is set in the last buffer of the output. The last_buf flag is set only for the main request since a subrequest’s last buffers does not end the entire output.

Body filters

The function ngx_http_output_filter(r, cl) invokes the body filter chain by calling the top body filter handler ngx_http_top_body_filter. It’s assumed that every body handler calls the next handler in chain until the final handler ngx_http_write_filter(r, cl) is called.

A body filter handler receives a chain of buffers. The handler is supposed to process the buffers and pass a possibly new chain to the next handler. It’s worth noting that the chain links ngx_chain_t of the incoming chain belong to the caller. They should never be reused or changed. Right after the handler completes, the caller can use its output chain links to keep track of the buffers it has sent. To save the buffer chain or to substitute some buffers before sending further, a handler should allocate its own chain links.

Following is the example of a simple body filter counting the number of body bytes. The result is available as the $counter variable which can be used in the access log.

Building filter modules

When writing a body or header filter, a special care should be taken of the filters order. There’s a number of header and body filters registered by nginx standard modules. It’s important to register a filter module in the right place in respect to other filters. Normally, filters are registered by modules in their postconfiguration handlers. The order in which filters are called is obviously the reverse of when they are registered.

A special slot HTTP_AUX_FILTER_MODULES for third-party filter modules is provided by nginx. To register a filter module in this slot, the ngx_module_type variable should be set to the value of HTTP_AUX_FILTER in module’s configuration.

The following example shows a filter module config file assuming it only has one source file ngx_http_foo_filter_module.c

Buffer reuse

When issuing or altering a stream of buffers, it’s often desirable to reuse the allocated buffers. A standard approach widely adopted in nginx code is to keep two buffer chains for this purpose: free and busy. The free chain keeps all free buffers. These buffers can be reused. The busy chain keeps all buffers sent by the current module which are still in use by some other filter handler. A buffer is considered in use if its size is greater than zero. Normally, when a buffer is consumed by a filter, its pos (or file_pos for a file buffer) is moved towards last (file_last for a file buffer). Once a buffer is completely consumed, it’s ready to be reused. To update the free chain with newly freed buffers, it’s enough to iterate over the busy chain and move the zero size buffers at the head of it to free. This operation is so common that there is a special function ngx_chain_update_chains(free, busy, out, tag) which does this. The function appends the output chain out to busy and moves free buffers from the top of busy to free. Only the buffers with the given tag are reused. This lets a module reuse only the buffers allocated by itself.

The following example is a body filter inserting the “foo” string before each incoming buffer. The new buffers allocated by the module are reused if possible. Note that for this example to work properly, it’s also required to set up a header filter and reset content_length_n to -1, which is beyond the scope of this section.

Load balancing

The ngx_http_upstream_module provides basic functionality to pass requests to remote servers. This functionality is used by modules that implement specific protocols, such as HTTP or FastCGI. The module also provides an interface for creating custom load balancing modules and implements a default round-robin balancing method.

Examples of modules that implement alternative load balancing methods are least_conn and hash. Note that these modules are actually implemented as extensions of the upstream module and share a lot of code, such as representation of a server group. The keepalive module is an example of an independent module, extending upstream functionality.

The ngx_http_upstream_module may be configured explicitly by placing the corresponding upstream block into the configuration file, or implicitly by using directives that accept a URL evaluated at some point to the list of servers, for example, proxy_pass. Only explicit configurations may use an alternative load balancing method. The upstream module configuration has its own directive context NGX_HTTP_UPS_CONF. The structure is defined as follows:

A module that implements a load balancing algorithm must set these methods and initialize private data. If init_upstream was not initialized during configuration parsing, ngx_http_upstream_module sets it to default ngx_http_upstream_init_round_robin.

init_upstream(cf, us) — configuration-time method responsible for initializing a group of servers and initializing the init() method in case of success. A typical load balancing module uses a list of servers in the upstream block to create some efficient data structure that it uses and saves own configuration to the data field.

init(r, us) — initializes per-request ngx_http_upstream_peer_t.peer (not to be confused with the ngx_http_upstream_srv_conf_t.peer described above which is per-upstream) structure that is used for load balancing. It will be passed as data argument to all callbacks that deal with server selection.

When nginx has to pass a request to another host for processing, it uses a configured load balancing method to obtain an address to connect to. The method is taken from the ngx_http_upstream_peer_t.peer object of type ngx_peer_connection_t:

sockaddr, socklen, name — address of an upstream server to connect to; this is the output parameter of a load balancing method

data — per-request load balancing method data; keeps the state of selection algorithm and usually includes the link to upstream configuration. It will be passed as an argument to all methods that deal with server selection (see below)

All methods accept at least two arguments: peer connection object pc and the data created by ngx_http_upstream_srv_conf_t.peer.init(). Note that in general case it may differ from pc.data due to “chaining” of load balancing modules.

get(pc, data) — the method is called when the upstream module is ready to pass a request to an upstream server and needs to know its address. The method is responsible to fill in the sockaddr, socklen, and name fields of ngx_peer_connection_t structure. The return value may be one of:

NGX_OK — server was selected

NGX_ERROR — internal error occurred

NGX_BUSY — there are no available servers at the moment. This can happen due to many reasons, such as: dynamic server group is empty, all servers in the group are in the failed state, all servers in the group are already handling the maximum number of connections or similar.

NGX_DONE — this is set by the keepalive module to indicate that the underlying connection was reused and there is no need to create a new connection to the upstream server.

free(pc, data, state) — the method is called when an upstream module has finished work with a particular server. The state argument is the status of upstream connection completion. This is a bitmask, the following values may be set: NGX_PEER_FAILED — this attempt is considered unsuccessful, NGX_PEER_NEXT — a special case with codes 403 and 404 (see link above), which are not considered a failure. NGX_PEER_KEEPALIVE. Also, tries counter is decremented by this method.

notify(pc, data, type) — currently unused in the OSS version.

set_session(pc, data) and save_session(pc, data) — SSL-specific methods that allow to cache sessions to upstream servers. The implementation is provided by the round-robin balancing method.